In the operation of Swiss-type screw machines, it is conventional practice to convert the necessary cutting and shaping operations to a contoured program cam which, as it rotates, controls the movement of one or more tools of the machine. In general, a radial inward cutting movement of a tool would be desirably programmed to proceed at a constant linear speed. To this end, the control cam for many conventional machines would have a surface contour in the form of an involute curve. In certain other conventional makes of Swiss-type screw machines, the orientation of the control cams, and the rocker arms which support the cutting tools, is such that the rocking of the arm will effect a slight repositioning of the cam follower in a circumferential direction, as well as in the desired radial direction. For such machines, the nominal involute curve normally desired is modified or "corrected" slightly to take into account the slight circumferential motion of the cam follower.
Heretofore, the laying out and machining of control cams for Swiss-type screw machines has been a slow and painstaking process, requiring highly skilled machinists. In addition, practical difficulties involved in the machining operation have limited the accuracy to which such cams could be .[.milled.]. .Iadd.cut .Iaddend.on a reasonably economical basis. Even with automatic, numerically controlled (N/C) equipment, it has been difficult and expensive to manufacture these control cams, because the numerically controlled systems available on a suitably economical basis have been limited to point-to-point .[.milling.]. .Iadd.cutting.Iaddend., resulting in a step-wise cutting action. With such equipment, after the numerically controlled cutting is completed, the cam must be further reworked in order to provide a smooth operating surface, and this tends to introduce inaccuracies, and also requires substantial time to be expended by a skilled machinist.
The need for smoothing out the abrupt transitions of the point-to-point cutting procedure has been long recognized, and efforts have been made to provide equipment capable of effecting a constant slope transition of the cutter from one set of coordinates to the next. The Theodore Weber U.S. Pat. Nos. 3,244,019; 3,248,624 and 3,426,619 are representative of prior attempt to adapt a constant slope control to a numerically controlled milling machine. These prior efforts have, however, been either inordinately complicated and expensive or lacking in performance effectiveness. In the case of the beforementioned Weber patents, for example, constant slope control is afforded by a mechanical variable speed and control, the adjustment of which is effected through the N/C control. However, because of the inherent inaccuracies in the ability to adjust the mechanical variable speed device, it is necessary to provide complicated supplementary control arrangements for making final compensating adjustments in the point-to-point positioning of the cutting tool at the end of each program stage. Since the practical usefulness of any kind of automated equipment involves an evaluation of its installation cost and operating expense in relation to man hours saved, extra cost and unreliability factors mitigate sharply against the commercial usefulness of such equipment. In this respect, up to the present time the industry has virtually dismissed the use of N/C equipment in the production of control cams for Swiss-type screw machines, because of the expense and difficulty of adapting such equipment to the high precision requirements of the cam cutting operation.